It seems like 2.2 (or the upgrade process) triggers in hibernate the
hallucination that I wanted to use bean validation (maybe it's just
the presence of javax.validation in the gwt servlet-dep jar, although
it was present in 2.1 as well).
If you don't want to use hibernate bean validation, you can add this
line to your hibernate persistence.xml file:
<validation-mode>NONE</validation-mode>
And all is well again.